There are a number of analog (but digital : ) o-scope modules that you can get your hands on for a few hundred dollars. (like this one:
http://www.muffwiggler.com/forum/viewto ... 6f78c1bdc5) Will not only show you voltage but also dual waveforms which is handy. But that's a fair sum of money that can be better spent (as you will soon see) on other modules.
But a simple multimeter or a voltmeter, possibly mounted behind a bland DIY panel with a single 1/4" or whatever jack type you like will do the trick. Then you can attenuate as somebody else said.
To give you just a bit more feedback.
You'll find that some modules leverage bipolar (to negative voltage, especially LFOs) while other just stay in the positive range. Some envelop generators also go negative. And btw: MoogerFoogers, despite saying "5V" actually push out greater voltage than that on the ring and also respond to high voltages coming back in (to Expression pedal jacks).
Many of the Moog circuits leverage the return voltage from Expression pedal jacks then 'add' this to the starting point of the pot on the panel (whether it be Voyager or Fooger). I'm not as technical so some of the other folks here but you can think of it as a base and an offset and I've found that beginning with a low base allow for greater range and values > 5V are actually useful in getting a given control to where you want it.
Certainly voltages > 10V are a bad idea and I 'think' there is protection at least at some level so best advice is to keep within range of what the equipment manufacturer recommends.
Gates are another matter. Some gear is extremely finicky when it comes to recognizing a gate either by leading edge, trailing edge, seeing a voltage at a specific level; many manufacturers (Synth.com is one) documents the voltages specifically. I've found Moog to be especially liberal on some pieces of gear and difficult on others (my Old School Voyager).
Hope this somewhat extended explanation helps.
Welcome to the club.
